This invention relates generally to a power circuit for an electronic timepiece using a battery as a power source and more particularly, to a power circuit which extends battery life by voltage reduction during normal operation and improves display quality during periods of heavy load. Because the capacity per unit volume of a lithium battery is larger than the capacity of a silver oxide battery, a quartz crystal timepiece using a lithium battery can have a longer battery life. The lithium battery has a disadvantage in that a large voltage drop occurs when a large current is flowing from the battery because of the high internal impedance of the lithium battery. This occurs at such times as sounding of an alarm or lighting a lamp in the timepiece.
A lithium battery for a timepiece usually has a voltage of approximately three volts. An oscillator circuit and a divider circuit are driven with a power source supplying 1.5 volts which is obtained by reducing the battery voltage to 1/2. The power source applying 1.5 volts is hereinafter referred to as V.sub.SS1/2.
In order to prevent voltage applied to a circuit driven with the V.sub.SS1/2 source from dropping below the voltage level which maintains oscillation in the oscillator circuit, power circuits have been devised which derive the V.sub.SS1/2 source directly from the battery, or a voltage regulation circuit operates to supply V.sub.SS1/2 with an output at the time of heavy load as when a lamp is lit or the like.
In conventional power circuits, although it is possible to prevent oscillation from stopping even when the battery voltage falls at the time of lighting a lamp of the like, there remains a disadvantage that lit segments in a multiplex driving liquid crystal display become darkened at the time of lighting of the lamp or the like. Such unsatisfactory performance becomes especially noticeable when the battery life is near its end or the ambient temperature is low.
What is needed is a power circuit for an electronic timepiece which conserves battery power during normal operation, and maintains timekeeping circuit performance as well as liquid crystal display quality when battery voltage is low.